Particulates or particulate matter entrained in exhaust (burned gas of diesel oil) from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction).
In order to suppress the particulates from being diffused into atmosphere, conventionally a filter for capturing particulates is incorporated into an engine exhaust system.
An example of the particulate filter comprises a honeycomb core made of ceramics such as cordierite and having a number of passages compartmentalized by porous thin walls, exhaust from an engine flowing through the passages.
In the above-mentioned particulate filter, alternate ones of the parallel passages have plugged one ends so as to guide the exhaust to unplugged one ends of the gas passages adjacent thereto; the passages through which the exhaust flows have the plugged other ends so as to connect unplugged other ends of the gas passages adjacent thereto to a muffler.
Thus, the particulates entrained in the exhaust are captured by the porous thin walls and only the exhaust passing through the walls is discharged to the atmosphere.
The particulates attached to the thin walls will spontaneously ignite to be oxidized when an engine operating status is shifted to a region with increased exhaust temperature.
However, for example, in a shuttle-bus running mainly on city roads with generally lower running speeds, there is few chance to continue an engine operational status capable of obtaining exhaust temperature suited for oxidation treatment of the particulates. As a result, a captured particulate amount will exceed an oxidized amount, leading to clogging of the porous thin walls.
Thus, recently, a plasma assisted exhaust emission control device (gas treatment reaction vessel) has been proposed which can oxidize particulates even if exhaust temperature is low (see, for example, Reference 1).
In this exhaust emission control device, inner and outer electrodes each in the form of drilled stainless cylinders are coaxially arranged in a chamber. A gap between the electrodes is charged with dielectrics in the form of pellets so as to allow the exhaust to pass. The exhaust from the engine is guided to a gap between the chamber and the outer electrode.
Thus, the particulates entrained in the exhaust supplied from between the chamber and the outer electrode to the pellet charged layer are attached to the pellets and only the exhaust passing through the layer is discharged to the atmosphere.
Higher voltage is applied across the electrodes to generate discharge plasma and excite the exhaust, so that unburned hydrocarbon, oxygen and nitrogen monoxide are activated into oxygen-containing hydrocarbon, ozone and nitrogen dioxide, respectively.
Thus, even with lower exhaust temperature, the particulates attached to the pellets will spontaneously ignite to be oxidized.
[Reference 1] JP 2002-501813A